With a wide area mobile communication represented by a cellular phone or the like, to reduce the output of a radio wave from the base station, one base station controls to cover a small divided communication area called “cell.” When power is turned on or when handover takes place, a mobile station such as a cellular phone can continue communication by selecting and using these cells (base stations) as appropriate. Selecting a cell by a mobile station is called a “cell search.” In a cell search, the mobile station detects an optimum cell to which the mobile station should be connected when power is turned on—that is, a cell having a minimum path loss. More specifically, cells are identified by peculiar scrambling codes, and cell search is performed by the mobile station by detecting a scrambling code of a cell which transmits a signal with maximum reception power in a downlink.
However, there is a problem that it takes a considerable time and computing load for the mobile station to identify many scrambling codes at one time, and there is therefore a growing demand for a technology which solves this problem.
Examples of conventional technologies related to cell search include a three-stage cell search scheme of the OFCDM (Orthogonal Frequency and Code Division Multiplexing) scheme (for example, see patent document 1).
According to the conventional technology described in patent document 1, scrambling codes can be detected at high speed by grouping the scrambling codes into several groups (for example, 32 groups). More specifically, in a second stage, a correlation between neighboring symbols is calculated to detect frame timing and a code group, and in a third stage, scrambling codes are identified from scrambling code candidates which belong to the code group detected in the second stage through a correlation calculation.
FIG. 1 shows a conventional frame configuration of OFCDM. As shown in FIG. 1, pilot symbols continue on a frame boundary in the time axis direction and a pilot symbol at the end of the frame is multiplied by a code group sequence indicating a group of scrambling codes.
FIG. 2 shows the conventional second stage processing of a cell search performed at the mobile station. The mobile station detects frame timing and a scrambling code group by calculating a correlation between a sequence extracted through a complex conjugate multiplication between neighboring symbols and a code group candidate. The code group and frame timing are detected at the same time through this correlation calculation among the neighboring pilot symbols.
Patent Document 1: Japanese Patent Application Laid-Open No. 2003-244763